Obtaining the ability of producing polyethylene resins having “broad” molecular weight distributions in a single reactor is one of the main directions of technological development in polyethylene catalyst field. Many research organizations and companies in the world have been studying and developing composite catalysts exhibiting different performances, in order to produce resins having “broad” molecular weight distributions in a single reactor.
U.S. Pat. No. 7,141,632 discloses a “metallocene-metallocene” composite catalyst including a good comonomer incorporating metallocene compound and a poor comonomer incorporating metallocene compound. Because the two metallocene compounds have substantially equal hydrogen response, it is possible to make use of the difference in the copolymerization ability to regulate molecular weight distribution. The produced polyethylene resins have improved mechanical and processing properties, and can be used in film application.
U.S. Pat. Nos. 5,614,456, 6,943,134 and 7,129,302 disclose “metallocene-Ziegler” composite catalysts. Under the same reaction conditions including the presence of comonomer and hydrogen, metallocene catalysts will give low molecular weight polymer having more branched chains due to their better copolymerization performance and hydrogen response, while Ziegler-Natta catalysts will give high molecular weight polymer having less branched chains due to their inferior copolymerization performance and hydrogen response. The mechanical properties of so-produced resins are not satisfactory.
Comparing Ziegler-Natta catalysts with chromium-based catalysts (also referred to as Phillips catalysts), Ziegler-Natta catalysts have better hydrogen response and inferior copolymerization performance, while chromium-based catalysts have excellent copolymerization performance but do not respond to hydrogen regulation. Thus, under the same polymerization conditions including the presence of comonomer and hydrogen, Ziegler-Natta catalysts will give polyethylene resins having extremely low to moderate molecular weights and higher densities, while chromium-based catalysts will give polyethylene resins having high molecular weights and lower densities. Unfortunately, the two catalyst systems may interfere with each other so that it is difficult to form a “Ziegler-chromium” composite catalyst on the same support. U.S. Pat. No. 6,828,268 discloses a process wherein a Ziegler-Natta catalyst and a chromium-based catalyst are used simultaneously in a single reactor. The particles of the two produced polymer resins are mixed merely “physically” but not at molecular level.